It is an inline "gear indicator" that will tell you the chain position on your rear cassette. Mine is positioned a few inches from the right brifter on the RD cable. There is a small red ball inside the clear tube that moves back and forth on the cable to your rear derailleur.

Last edited by Old School; 05-22-09 at 09:24 PM.

Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW! WHAT A RIDE!"

When I switched from my mountain bike shimano setup with a thumb shifter to the "new" 9 speed Jamis Nova, I complained to my lbs that I missed the visual que of the thumb shifter. They installed one of those inline gear indicators.

As I got used to the 9 speed I realized that all you have to do is look at the gears. In any case, the plastic starts getting UV damage and it gets harder and harder to see anything. And when I recabled I just left it off.

It is a good visual aide for newbies learning how to use the new gears.

I had some for my XTR DC shifters and thought, boy these are stupid so I never put them on.
I just got a road bike with Dura Ace shifters and this was already installed, and I really like it.
I always looked down at the rear derailleur to see what gear I was in, but find this way better. Old habits die hard and I still find myself looking at the cassette, but less and less.

I have one of these on each road bike that my wife and I own and one on the tandem. A bike is not complete without one of them. 9 and 10-speed versions are easily available from local bike stores, and sometimes they'll have an old 8-speed version lying in the back of a drawer.

A lot of people who care more about fashion than function say that you don't need one - but life is certainly easier with one, and it weighs less than 5 grams, so why not?

I find it very odd that it comes as standard part with Dura Ace brifters, and I believe there is a similar thing integrated into the top of the current Sora and Tiagra brifters, but people who buy 105 or Ultegra brifters get nothing - we have to buy the Dura Ace model from the LBS (very few online stores carry them).